The “alignment” refers to the alignment of wheels, that is, the angle of the wheels with respect to the traveling direction of a vehicle or the road surface. The alignment is mainly represented by a camber angle and a toe angle. Misalignment is caused by displacement of a suspension apparatus due to an impact that is applied when a vehicle is traveling, deterioration of tires due to aging, and the like, and affects traveling stability of the vehicle, uneven wear of the tires, and the like. Thus, the alignment is measured and adjusted by various apparatuses.
The alignment is typically measured and adjusted by measuring the angle and dimensions of each wheel, and adjusting the toe angle and the camber angle respectively to target values that are determined when designing vehicles.
For example, Patent Document 1 describes a technique for improving measurement accuracy of the alignment. In this technique, a pair of non-contact distance measuring sensors are provided so as to face a side surface of a tire to be measured. With the distance measuring sensors being kept in a stationary state, the tire is rotated by 360 degrees to continuously measure the distance to the side surface of the tire. Then, with the tire being kept in a stationary state, the distance measuring sensors are rotated by 360 degrees to continuously measure the distance to the side surface of the tire. A predetermined calculation is performed based on the distance data obtained by rotating the tire, and the distance data obtained by rotating the distance measuring sensors, whereby measurement accuracy of the alignment is improved.
However, this technique has the following problem. This technique is intended only to measure and adjust the alignment when the vehicle is stopped, and is not intended to measure the alignment when the vehicle is traveling. Thus, it is difficult to keep track of an actual traveling state of the vehicle, and adjustment cannot be made accurately.
That is, tires during traveling of the vehicle have characteristics such as deformation of the tire caused by a lateral force that is generated due to a slip angle, camber thrust that is generated due to the camber angle, and the relation between these forces and the rigidity of the tire, and non-uniformity due to a manufacturing error. Thus, in order to accurately measure and adjust the alignment in view of the traveling state of the vehicle, the alignment needs to be measured and adjusted based on these characteristics of the tires.
For example, Patent Document 2 describes a technique of measuring and adjusting the alignment based on characteristics of tires. In this technique, two or more roller pairs are used to drive wheels (tires), and forces respectively generated on the rollers are measured, and the toe angle and the camber angle are measured based on the directions of the measured forces.    Patent Document 1: Japanese Patent Application Publication No, JP-A-H06-11420    Patent Document 2: Japanese Patent Publication No. JP-B-S51-1868